Organic light emitting diodes (OLEDs), sometimes referred to as lamps, are desirable for use in electronic media because of their thin profile, low weight, capability of producing a wide variety of emission colors, and low driving voltage. OLEDs have potential use in applications such as backlighting of graphics, pixelated displays, and large emissive graphics.
OLEDs typically consist of an emissive organic element sandwiched between two electrodes: a cathode and an anode. The emissive organic element may be a single layer or it may be a multilayer. The emissive element must be capable of electron transport and hole transport as well as light emission. Charge carriers, i.e., electrons and holes, are injected into the emissive element from the cathode and anode, respectively. Electrons are negatively charged atomic particles and holes are vacant electron energy states that behave as though they are positively charged particles. The charge carriers migrate towards the center of the emissive element where they combine to emit light.
The emissive element may be a molecularly doped polymer where charge carrying and/or emitting species are dispersed in a polymer matrix (see J. Kido, “Organic Electroluminescent devices Based on Polymeric Materials,” Trends in Polymer Science, 1994, 2, 350-355), a conjugated polymer where layers of polymers such as poly(phenylenevinylene) act as the charge carrying and emitting species (see J. J. M. Halls, D. R. Baigent, F. Cacialli, N. C. Greenham, R. H. Friend, S. C. Moratti, and A. B. Holmes, “Light-emitting and Photoconductive Diodes Fabricated with Conjugated Polymers,” Thin Solid Films, 1996, 276, 13-20), a vapor deposited small molecule heterostructure (see C. H. Chen, J. Shi, and C. W. Tang, “Recent Developments in Molecular Organic Electroluminescent Materials,” Macromolecular Symposia, 1997, 125, 1-48), or various combinations of these elements.
OLEDs generally require some level of patterning to be of practical use. This could be as simple as patterning the anode and/or cathode into a simple shape, such as a rectangle, for use as a backlight or as complex as a multitude of small pixels, each composed of red, green, and blue subpixels, for a full-color, high-resolution display.
Transparent anodes may be patterned by etching away portions of the conductive anode material with acid through a photoresist mask. Alternatively, insulating coatings, such as alumina, may be applied to the anode through a shadow mask to create a patterned anode. Metal cathodes are typically patterned by vapor deposition of the metal through a shadow mask placed onto the device being fabricated. The organic layers may be patterned by, for example, evaporation through a shadow mask, ink jet printing, or laser-induced thermal imaging (LITI). All of the typical patterning techniques currently in use for OLEDs must be done prior to or during the device fabrication and before the device is encapsulated. Thus there are special handling requirements, such as high vacuums and inert atmospheres, for these patterning techniques that preclude their use outside of a sophisticated OLED fabrication facility.
Transparent graphics could be applied to an OLED device to create a patterned emissive element after device fabrication and packaging. However, such an emissive element could suffer from parallax and would generally be less power efficient since portions of the light generated by the device would be blocked by the graphic overlay.